The present invention relates to a quantum encryption device. It can be applied especially to the production of encryption systems on an industrial scale.
There are known ways of making transmission lines protected against indiscreet listening by using the properties of quantum uncertainty. A sender, commonly known as Alice, sends a message to a receiver, commonly called Bob, in form of discrete quantum states. If an eavesdropper, usually called Eve, intercepts and measures these states, Alice and Bob can make carry out checks, on a public means, on the action introduced by Eve""s measurement. Furthermore, through a play of random diagonal bases, it can be shown that the channel available to Alice a posteriori, namely after transmission, through a public means, of the bases used for transmission, has a capacity greater than the capacity available to Eve a priori. In other words, quantum encryption plays on the impossibility of observing two persons by sharing a code. A quantum code is therefore used to define an encryption code by using properties of non-observation which mean, essentially, that Alice cannot share the code with Bob. Quantum encryption consists especially of the transmission of information by isolated photons to which an encoding is applied. One problem especially is to be able to detect these isolated photons.
The quantum phenomena used are generally in the field of optics, because it is in this field that the quantum phenomena are most observable. Thus, there are known systems in which the quantum state used is the polarization or phase of a photon on an optical line. Such systems require a very high quality detector to give a reasonable chance of detecting the unique photons that convey the information elements. These detectors are complex to make and costly. In fact, they are not suited to industrial-scale and mass applications such as for example business transactions using electronic and/or optical channels. For example, CCD cameras of the large-scale consumer type has a noise level of about 30 to 300 photons. It is therefore not capable of detecting a few photons. By contrast, laboratory CCD cameras designed for example for astronomical applications are capable of carrying out dissociations at the photon level, but do so at the cost of extremely complex and costly modes of implementation. In particular, by construction, the CCD cameras cannot or can hardly go below a noise level in the range of ambient temperatures. This can easily be shown by a few known relationships and variables. If Q is the minimum electrical charge that a CCD camera can detect and C the capacitance of its detection components, these two parameters Q and C verify the following relationship:                                           Q            2                    C                =        kT                            (        1        )            
where kT is substantially equal to 0.4.10xe2x88x9220 Joules at ambient temperature.
Furthermore, the capacitance C is in the range of 10xe2x88x9213 F., and therefore the above-mentioned charge Q will be substantially equal to 2.10xe2x88x9217 Cb, giving about 120 times 1.6.10xe2x88x9219 Cb, this latter charge corresponding to the electrical charge of a photon. At ambient temperature, a CCD detector cannot detect less than 120 photons approximately. The square root of the number of photons represents the noise of the detector. It is of course possible to reduce this noise by increasing the complexity especially of the detection by known correlation functions. However, the remaining noise level is always appreciably greater than a few photons, unless we go to very low temperatures, which is feasible neither industrially nor economically. Other types of detectors such as for example PIN diode detectors or avalanche detectors have similar detection characteristics.
A main aim of the invention is to enable the making of a quantum encryption device using detectors and amplifiers that are standard devices, hence low-priced and available in the market. To this end, an object of the invention is a quantum encryption device comprising a transmitter and a receiver between which an encrypted piece of information is transmitted, wherein the transmitter comprises at least:
means to send a packet of photons and an information photon to the receiver, the appearance of the information photon following a law of probability s and being lagged with respect to the packet by a period of time xcfx84;
means for the encoding of the information photon; and wherein the receiver comprises at least:
means for the encoding of a second packet of photons coming from the first packet and lagged substantially by the period of time xcfx84 with respect to this first packet;
interference means to obtain the interference of an information photon with the second packet of photons, the interference means providing a packet of photons to create a quantity of photons substantially equal to xc2x12 times the variance a of the noise due to the second packet.